In recent years, a substantial amount of effort has been devoted to improving short circuiting arc welding by controlling the various portions of the welding cycles, each constituting a short circuit condition followed by an arcing condition. During the short circuit condition, a molten metal ball formed on the end of the advancing welding wire engages the molten metal pool on the workpiece causing a high current flow through the consumable welding wire and molten metal ball. This short circuit condition is terminated by an electrical pinch action causing the metal forming the molten ball on the wire to electrically constrict and then break away from the welding wire in an explosion type action often referred to as a "fuse" or "the fuse". Controlling current flow during the short circuit portion of the welding cycle is accomplished by the power supply control circuit. In addition, a premonition circuit is usually provided so that a given increase in dv/dt indicates an imminent formation of the fuse. Consequently, the welding current can be dropped to a background level I.sub.B, or to a lower value, immediately before the fuse occurs. In this fashion, the energy of the fuse during each welding cycle is drastically reduced. This reduces spatter at the termination of the short circuit condition. Various circuits for controlling the current flow during the short circuit portion or condition of the welding cycle are known in the art as spatter control circuits since the fuse is considered to be a primary source of spatter during short circuiting arc welding. Other spatter producing dynamics of the welding process have been recognized by applicant and prior patents have provided novel control concepts to reduce spatter caused by such dynamics. One concept was to provide a high energy pulse following a slight time delay after the fuse so that the arcing condition subsequent to the fuse could be initiated by a high energy current pulse sometimes referred to as a "plasma boost" pulse. By using a high energy plasma boost current pulse immediately upon initiation of an arcing condition in the welding cycle, melting by anode heating at the tip of the welding wire being fed toward the molten metal pool on the workpiece occurred rapidly. This rapid melting allowed formation of a molten metal ball on the end of the wire of a uniform size, which ball was then moved toward the pool of molten metal as the wire was fed toward the workpiece. After the plasma boost pulse of current, a plasma condition was created by reducing the welding current to a level below the plasma boost level, but greater than the background current level. This reduced current mode was then terminated and a background current I.sub.B was passed through the arc to maintain the molten condition of the molten ball until the next short. By controlling the current level and using a fixed time for the plasma boost pulse, the energy in the plasma boost pulse was regulated. The end of the wire was melted to form a molten metal ball having a somewhat uniform size based upon an amount of energy applied during the plasma boost current pulse and then the plasma portion. After the plasma portion, the arc was operated at a background current level maintaining a molten condition until the short circuit occurred.
In the past, the amount of energy introduced into the wire during the arcing condition of the welding cycle has been controlled to produce a selected amount of melting energy. The plasma boost and subsequent plasma condition was controlled in a manner to control the amount of energy introduced into the wire as it was being melted preparatory to being shorted and transferred by surface tension from the welding wire to the molten metal bath on the workpiece. It has been suggested to provide a constant watt control for the plasma boost portion of the plasma employed to melt the advancing welding wire. To accomplish this, in the past, the joules were measured during the pinch cycle, plasma boost portion and the arc plasma portion of the welding cycle. When the joules accumulated to a certain level, the plasma portion of the cycle was terminated and the current was held at the background level awaiting a subsequent short circuiting condition. Thus, a fixed energy accumulation occurred during each plasma boost and plasma portion of the welding cycle by terminating the plasma time when the measurement of accumulated energy reached a certain level. This process worked extremely well in the field. However, changes in the wire extension are reflected as relatively small changes in the total energy introduced during a given welding cycle. Consequently, it is difficult to accurately control the welding current to compensate for relatively small changes in the wire extension from one welding cycle to the next. Thus, accuracy of the applied energy varied with changes in extension or stick out.
In addition, the prior spatter control systems, which are vastly improved over other systems, involves a rapid decrease in the current between the plasma boost portion of the heating cycle and the plasma portion, which second portion is subsequently terminated when the accumulated energy reaches a given level. This rapid decrease in welding current flow from the plasma boost portion to the plasma portion can cause certain agitation of the weld puddle that can result in inadvertent shorting and other spatter producing mechanics. Further, when such shorting occurs during the plasma portion of the cycle, the pinch circuit control is initiated at once.
This pinch control operation will cause current flow at a high level resulting in a rapid separation of the metal joined during this inadvertent short. Such rapid separation of the metal occurs before the premonition circuit (dv/dt circuit) is initiated; therefore, the fuse is not detected and there is no immediate drop in the welding current prior to metal separation. Consequently, such metal separation is at high current and can result in a certain amount of spatter.
The present invention relates to an apparatus and method for controlling the spatter in a welding process of a short circuiting type welding operation and it will be described with respect to this type of welding; however, the invention has certain broader applications which will be apparent during the description of the invention. The present invention relates to a further refinement of a spatter control system of the type described in the patents incorporated by reference herein and overcomes the specific situations described in the introductory portion of this disclosure.
In accordance with one aspect of the invention, compensation is provided for small changes in the welding wire extension which can result in slight differences in the melting energy introduced into the welding wire during the plasma portion of the welding cycle and, thus, cause some differences in the ball size. In accordance with this aspect, the voltage drop in the wire extension is measured during the early portion of the pinch portion of the welding cycle. This measurement of the extension voltage is accomplished when the pinch current is near the same magnitude during each successive welding cycle and is done over a very short time, less than 500 microseconds. As the wire extension changes during the welding process, the difference in the extension length is reflected as a change in the measured voltage during the pinch portion of the welding cycle. This short duration voltage measurement is taken when the current is nearly uniform and produces a series of voltage spikes having a short duration and a voltage magnitude equal to the product of applied pinch current and electrode extension in terms of resistance. The magnitude of the measured voltage level is proportional to the actual extension of the wire. The level is recorded by averaging the voltage levels of the thin voltage spikes during subsequent short circuit cycles. The average voltage level or value is a measurement of the wire extension length and is the voltage across the extension during the welding cycle. This extension length in voltage form is multiplied by the current during the plasma boost pulse so that the wattage during a plasma boost pulse is controlled by the averaged extension voltage. The product of the extension voltage and the current during the plasma boost portion of the welding cycle is then compared to a reference value for the wire wattage for the purpose of controlling the welding current. This concept maintains a constant electrode extension wattage during each subsequent plasma boost portion of the welding cycle. Consequently, the same size ball is melted and produced on the end of the wire during each plasma boost portion regardless of variations in the extension of the wire from the wire holder. This type of circuit is an improvement over prior spatter control systems and substantially enhances the ease of welding, while still maintaining the desired spatter control. The operator adjusts the reference wattage prior to a welding procedure by holding the extension at the desired value during initial welding. The current flow during the plasma boost portion of the welding cycle is controlled to maintain the actual wattage in the welding wire to a desired level, irrespective of variations in the extension length.
In accordance with another aspect of the invention, the reference wattage for the plasma boost is created by employing the same thin voltage spike measuring concept for determining the wire extension voltage over a relatively short time and at a precise location during the pinch portion of the welding cycle. At this short condition, the voltage is the extension or stick out voltage. When an operator initially starts the welding run or procedure, the welding current during the plasma boost portion of the welding cycle is controlled to obtain a fixed current during the power boost portion. However, the actual measured voltage levels indicative of the actual extension length at the start up of the procedure are stored and saved during this initial start up process. Thus, while the operator maintains the desired stick out during the initial start up, the reference wattage value is created by multiplying the stored voltage spikes indicative of the desired extension by the subsequent plasma boost currents, averaging and storing this product as the reference wattage value. Thereafter, this initially created reference wattage is employed to control the welding current during the plasma boost portion of the welding cycle for the remainder of the run, in accordance with the averaged sample voltage levels measuring the actual wire extension at each pinch pulse, to maintain constant extension wattage during plasma boost. Since the voltage measurement both for the reference wattage start up procedure and the continuous welding operation is made during a short circuit condition, the measured voltage magnitude is indicative of the voltage in the wire during the short. This voltage changes according to the extension or stick out of the welding wire.
In accordance with another aspect of the present invention, a capacitor is charged to a voltage proportional to the general current level during the plasma boost portion of the welding cycle. After the plasma boost portion, the plasma portion is initiated. At this time, a rheostat discharges the charged capacitor and the current level of the plasma portion follows a time constant curve created by this discharging action. This discharge action causes a voltage to decay along a time constant curve from a high level indicative of the current level during the plasma boost to a low level during the plasma portion. The control system causes the welding current to flow in a curve matching this decaying voltage curve. In this manner, there is a decay of the current after the plasma boost portion to prevent a rapid decrease in the current between these two arc portions of the melting operation for the wire. This procedure reduces the puddle agitation by slowing reducing the welding current instead of abruptly reducing the welding current as previously done after the plasma boost. Instantaneous reduction of the welding current after the plasma boost portion and before the plasma portion caused changes in the pressure exerted on the welding puddle and caused a certain amount of metal agitation. By reducing the current in a slow, continuous decay, the arc pressure on the melted pool is slowly reduced. This action dampens the weld puddle agitation. The time constant for the current decay can be changed by adjusting the rheostat. The plasma portion of the cycle can be instantaneously switched to the background current level in an abrupt fashion to terminate the plasma portion when the melting energy reaches a certain level. This switch action is done at a lower current level and does not cause agitation of the molten metal in the puddle. By providing a maximum time for the plasma portion, the plasma portion of the welding cycle does not have an excessively long decay time, which could occur if a high time constant were employed in the decay circuit. This aspect of the present invention gradually reduces the plasma current and maintains a quiescent pool to reduce the likelihood of an inadvertent short circuit.
In accordance with another aspect of the present invention, there is provided an arrangement for dealing with an inadvertent short circuit during the plasma boost or plasma portion of the welding cycle. It is possible that during the arc condition of the welding cycle a short can occur. This short is sensed by an abrupt decrease in arc voltage, which detection initiates the pinch pulse including its initial delay. Since the metal contact is caused by a movement of the puddle to contact the electrode, during the initial delay of the pinch cycle, gravity tends to pull the metal away from the electrode. This can cause a reduced metal contact area during this inadvertent short. The metal may be separated quite rapidly by the high current flow to create a fuse before the premonition dv/dt circuit can be initiated at the normal break point. This could cause severe puddle agitation by the break of the premature short. This premature short can occur when a portion of the weld puddle rises and contacts the molten metal of the end of the welding wire. Surface tension and gravity then abruptly pull the shorted, melted mass of metal back into the weld puddle. The surface tension breaks the fuse before the premonition circuit can be activated. This causes spatter.
In accordance with another aspect of the present invention, a premature, inadvertent short condition is detected by an increased voltage. If it is determined that this increased voltage is not a normal short, the break point of the pinch pulse is immediately lowered to a substantial current level, such as about 20% of the original break point current. By reducing the break point current, the pinch pulse shifts to the gradual current slope as soon as the pinch current increases to the preselected lower level, such as 20% of the normal maximum pinch current. Thus, a longer time is experience before separation of the metal and the premonition circuit can become active and allow control of the Darlington switches at the metal fuse to eliminate spatter caused by a high current break or fuse.
The primary object of the present invention is the provision of a method and apparatus for maintaining a generally constant wattage in the welding wire of a welding operation during the plasma boost portion of the welding cycle as the extension or stick out distance changes. In accordance with this object, an operator may manipulate a semi-automatic welder operating in the short circuiting mode in a manner changing the extension distance or stick out of the wire without substantially changing the operating characteristics of a spatter control system.
Yet another object of the present invention is the provision of a method and apparatus for maintaining a generally constant wattage in the wire forming the extension or stick out of a welding operation by creating an extension reference wattage, measuring the actual extension wattage at any given time during the welding procedure, comparing the measured wattage with the reference wattage to determine an error signal and using this error signal for controlling the welding current during the plasma boost portion of the welding cycle so that the plasma boost wattage is controlled at the reference wattage by controlling the welding current during the plasma boost portion of the welding cycle.
Still a further object of the present invention is the provision of a method and apparatus, as defined above, which method and apparatus involves creating a voltage spike during the pinch portion of the welding cycle and averaging these voltage spikes to give the voltage component for both the reference wattage and the instantaneous wattage. A comparison of signals representing these wattages are used to control the welding current during the power boost portion of the cycle.
Another object of the present invention is the provision of a method and apparatus, as defined above, which method and apparatus averages the measured extension voltage over several cycles for the purpose of creating a voltage level representing a reference extension wattage. This average also creates an continuously updates a voltage level representing the actual instantaneous wattage during the power boost portion of the welding cycle.
Still a further object of the present invention is the provision of a method and apparatus, as defined above, which method and apparatus controls the wattage directed to the welding wire during each welding cycle to control the size of the ball melted at the end of the welding wire during each plasma boost portion of the welding cycle, regardless of the extension or stick out length of the welding wire.
Another object of the present invention is the provision of a method and apparatus, as defined above, which method and apparatus substantially increases the ease of welding in a spatter control system allowing the operator to have certain latitude in positioning the holder for the welding wire during a given welding procedure or welding run.
Still another object of the present invention is the provision of method and apparatus for controlling the welding current level following the plasma boost portion of a welding cycle for spatter control, which method and apparatus causes the plasma current after the plasma boost pulse to gradually decrease or decay from the high plasma boost current level (such as about 300 amperes) toward the background welding current level (such as about 20 amperes).
Still a further object of the present invention is the provision of a method and apparatus as defined above, which method and apparatus controls the current level after the plasma boost pulse so that the welding current does not abruptly change from a generally high level value to a generally low level value, which abrupt change can cause a certain amount of pool agitation.
Yet a further object of the present invention is the provision of a method and apparatus, as defined above, which method and apparatus controls the welding current level after the power boost portion of the welding cycle in a manner to control the agitation of the molten metal pool immediately after the power boost portion of the welding cycle.
Yet another object of the present invention is the provision of a method and apparatus for controlling the break point of a pinch pulse of the welding cycle used in spatter control, which method and apparatus detects an inadvertent short during the arcing condition and immediately shifts the break point of the pinch pulse to a level substantially below the normal 60% break point current level. In accordance with this object, the preferred embodiment reduces the break point current to approximately 20% of the maximum pinch pulse current normally experienced in the welding cycle.
Yet another object of the present invention is the provision of an improved spatter control system for a short circuiting welding operation, which system may be employed for other welding operations and for the purpose of adjusting the stick out or extension in an automatic system, such as a robot control system.
These and other objects and advantages will become apparent from the following description of the present invention taken together with the drawings described below: